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UK cars are calibrated in MPH. For continental driving ease I always use my Tom Tom calibrated in KM.
In pre Tom Tom days I rembered the rule of 8 to 5; i.e. 80 Km = 50 MPH, 50 Km = 30 MPH, etc.
A little rough may be, but in 40 years of UK and continental driving I not received a speeding ticket when it was necessary to carry out a mental conversion!

There isn't a way to change the cruise control speed (if that's what you're talking about) to mph except through coding the car itself to work in Miles for the units of measure. And by coding I mean the same process folks have been using to delete the front amber blinkers as parking lights, delete the iDrive disclaimer screen, etc.....